The most prevailing form of color photography is the so-called nega/paper system (hereinafter referred to as "N/P system"), that is, a system of obtaining color prints by taking pictures with color negative films, and after processing the films, printing the images of the processed film on photographic papers in a photofinishing laboratory. This system has gained international popularity, and has become firmly established in the current market because it can offer convenience and various careful services to customers.
In this system, color prints have been obtained by subjecting a color negative film to conventional processing, printing the image obtained in each frame on a positive material, such as color paper, by giving thereto uniform exposure in accordance with a printing process called non-scanning area-wise exposure, and then performing development.
In recent years, on the other hand, the images of a color negative film have been used in various ways. For instance, such images are photoelectrically read with a scanner to convert the image information into electric signals, subjected to proper picture processing, and then printed or exposed to other image media. According to our analysis, however, the photographic characteristics of current color negative films are not always adapted for new systems of image utilization which involve digital picture processing, because those films are designed for the foregoing non-scanning area-wise exposure system.
The most remarkable inadequacy is attributable to the difference in influence of mask density. The current color negative films are designed so as to have high mask densities, so that the density of the non-image part (hereinafter referred to as "Dmin") is high. Since the gradation part is on the non-image part, the density value in the high density portion approaches to 4. In the system which comprises reading image information with a scanner and applying some picture processing to the read information, too high density value of the high exposure portion in the image part results in a lowering of the reading accuracy of an image density reading device, so that the readable density has its limits.
In the case of high mask density, therefore, the difference between the readable highest density and Dmin becomes small, and thereby the range of exposure amounts to reproduce the gradation in a color positive material, namely a photographic latitude (which is synonymous with "a dynamic range"), is lessened. In the conventional system which requires no picture processing, it is necessary to make corrections on the side absorptions of color-developed dyes by sufficient masking in order to ensure a satisfactory level of saturation to the images reproduced in a color positive material. When the non-scanning area-wise exposure system is adopted for printing, the extension of a printing time can cope with a high density portion because of high exposure in color negative film, so that the conventional system is not severely restricted by the raise in density due to masking. Further, the inventor of the present invention has found that the saturation can be heightened electrically by performing digital picture processing; as a result, it becomes unnecessary to raise the mask density for masking.
The term "density" as used herein stands for photographic density, which is synonymous with optical density. Thus, the term "density" used in the present invention refers to the density generally used by one skilled in the arts.
Additionally, a supplementary explanation is made below as to the terms masking used in connection with color negative films: The magenta dyes formed by color development in color negative films have side absorption in the neighborhood of 440 nm in addition to spectral absorption at essentially required wavelengths, namely in the vicinity of 550 nm. This side absorption is responsible for a drop in the saturation of color print images. As a measure against such a saturation drop, a magenta coupler having such a coupling release group as to absorb light in the spectral region of the side absorption (a colored group containing an arylazo group), namely a coupler with mask-function, is incorporated in a color negative film. The coupler with mask-function is designed so as to adjust the balance between a density raise caused by the side absorption of the magenta dye formed upon color development in the vicinity of 420 to 440 nm and a density drop resulting from a decrease of light absorption in the same spectral region due to the elimination of the foregoing coupling release group from the magenta coupler, thereby masking the color turbidity arising from the side absorption. The term masking refers to the compensation mechanism mentioned above. Accordingly, the masking is essential to the conventional N/P system.
Reduction in photographic latitude (dynamic range) is attributable to other causes besides the high mask density. In particular, the reduction tends to occur when the quality of development processing is lowered by variations in processing steps and some other causes. For instance, stains (Dmin) increased by color contamination (due to insufficiency in washing out spectral sensitizers and antihalation dyes) and bleach fog (undesirable dye stain generated by the oxidative action of a bleaching agent) cause the photographic latitude reduction problem.
On the other hand, desilvering inadequacy is liable to occur when development processing is carried out under an unsuited condition resulting, e.g., from variations in processing steps. In such a case, the quantity of silver residue is especially large in high exposure section, so that the high density areas have a further heightened density. The reading accuracy on the high density side deteriorates when the density value is increased beyond 3.5. This error in reading accuracy is reflected in the highlight areas of a print to exert a bad influence on the important part of printed image qualities. Accordingly, it is necessary to have a high maximum density (Dmax) on the characteristic curve from the viewpoint of improving the image quality, but in the case of too high Dmax the image deterioration factor will function due to unsatisfactory reading accuracy. Thus, it is desirable that Dmax be lowered to the level on which reliable reading become possible as far as the exposure range is within the photographic latitude and still the information recorded in a color negative film can be read exactly.
Additionally, the term "maximum density" is used in two senses, namely "maximum density" as the maintenance limit of accuracy in reading densities and "maximum density" on the characteristic curve of a color negative film. In places where fears are entertained that some confusion will arise, the former is referred to as "(reading) limit density" and the latter is referred to as "Dmax". Making additional remark, most of color negative films have a long gradation range, and so they have indefinite Dmax in contrast to other photographic materials. Thus, Dmax used for a color negative film is intended to describe the density in the upper limit region of the characteristic curve.
As mentioned above, color negative films designed on the assumption that the images formed therein are printed on positive materials by directly undergoing the non-scanning area-wise exposure system do not necessarily have most appropriate designs for the system where the image information is read by a scanner, digitized and then transmitted to various kinds of printing materials and other media; therefore, in the application to such a system, they generally develop defects that the photographic latitude is lessened by their high Dmin and the resultant high image density in high exposure section and the image quality deteriorates due to faults in reading over negative image information. Thus, it has been expected to develop an image formation method which can solve the aforementioned problems confronting the use of digital picture processing.